1. Field of the Invention
The present invention relates generally to Micro-Electrical-Mechanical System (MEMS) technology. More particularly, it concerns improving the useful traveling range of MEMS actuators.
2. Description of Related Art
Micro-electrical-mechanical systems (MEMS) often use actuators to impart motion to, for example, positioning optical devices and switches and for turning gears. Electrostatic actuators are one of several types of actuators used in MEMS, including magnetic piezoelectric, thermal, and optical actuators. When compared to other micro actuators, electrostatic actuators generate force of several micro Newtons (μN) and consume virtually no electrical power.
One of the most common electrostatic actuators is a comb drive, which generates a force dependent on the square of the applied voltage. The main issue of the comb drive design is achieving large deflections while minimizing the actuation voltage, resulting in a small deflection-to-size ratio of the actuator. These requirements are typically met by balancing the design of the actuator's suspension and varying the size of the force-generating comb structure. However, comb drives inherently suffer from an electromechanical instability called side pull-in or lateral instability, as illustrated in FIG. 1A. Lateral instability occurs when the electrostatic stiffness transverse to the axial direction of motion exceeds the transverse mechanical stiffness of the suspension. Additionally, although the comb structure is fabricated to be perfectly symmetrical, the comb structure is always unbalanced, causing the neighboring electrodes to contact each other when the voltage-deflection conditions are favorable. Weak suspensions and large forces, designed to achieve large traveling ranges, increase the lateral instability.
To overcome this, the transverse stiffness of the suspension is generally increased. Unfortunately, current suspension stiffening techniques limit the traveling range of the actuator. For example, fabrication techniques such as deep reactive ion etching (DRIE) have allowed the comb structure to be fabricated in single crystal silicon with typical thicknesses of several tenths of microns. These thicker structures can provide larger vertical electrode areas and substantially higher stiffness, but do not improve the limited deflection of comb drive structures.
The shortcoming of conventional methodologies are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning MEMS actuators. Other noteworthy problems may also exist, however, those mentioned here are sufficient to demonstrate the methodologies appearing in the art have not been altogether satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.